The present invention relates to a lead-coated heat transfer tube and, more particularly, to a heat transfer tube having an inner tube such as a copper tube or a steel tube and an outer tube such as a lead tube or a lead alloy tube.
A heat transfer tube formed of a copper or steel tube used for a heat exchange to recover heat energy exhausted from dirty gas such as heavy oil combustion exhaust gas is known in general to be corroded due to sulfuric acid contained in the gas in a short time. Thus, a double tube in which a copper tube or an outer tube coated with a lead alloy provided on the outside of a steel tube has heretofore used. In the conventional heat transfer tube of this type, an air layer can form between the inner tube and the outer tube. If the air layer is thus formed, the heat conduction characteristic of the tube is deteriorated, and when the outer surface of the outer tube is in contact with exhaust gas at approx. 280.degree. C. or higher, even if cooler water is flowed in the inner tube, the temperature of the outer tube reaches the same temperature as the exhaust gas temperature, with the result that the outer tube locally expands, cracks or becomes softened and melted. Thus, the conventional heat transfer tube is not satisfactory.